There is a strong need for fiber lasers because of their ease of use, low operating cost, versatility and high beam quality they provide. However, several problems hamper their even wider use, especially for high power applications. First, the assembly of fiber lasers and amplifiers still requires highly qualified professionals and a large amount of manual handling. Second, at high power levels, impurities or doping elements in the fiber lead to fiber darkening, which finally result in fiber damage and system failure. Third, even in non-doped fibers, intrinsic silica optical nonlinearity leads to different nonlinear effects such as Brillouin scattering. These nonlinear effects prevent light from propagating in the fiber and result in system failure.
In order to improve the current situation, fiber laser manufacturers employ different techniques, such as material improvement (e.g., reduction of the nonlinear coefficient, reduction of photo-darkening through suitable fiber treatment), and fiber improvement (e.g., large core area fibers allowing reducing the power density in the fiber core; photonic crystal fibers where part of the field is located in air pores within the fiber).
The relative high cost of fiber lasers manufacturing is still not solved, and the cost per watt of such lasers remains among the highest in the industry.
An alternative that has been developed by several groups is the replacement of the fiber by a planar waveguide. Planar waveguides usually suffer from higher losses but present the advantage of low fabrication cost since it is possible to use standard microelectronic processes in order to manufacture them. Using standard processes, it is possible to grow silica films on different types of substrates. However, in this case the silica layer is limited to a few microns thickness and the resulting waveguides cores have a small cross-section, which prevent using them for high power applications.
It has been suggested (Masaki Kohtoku et al, New waveguide fabrication techniques for next-generation PLC, NTT technical review, vol. 3, no 7, July 2005) that thick layers of silica can be obtained if a flame hydrolysis deposition (FHD) technique is used. FHD is a standard process for obtaining fiber pre-forms and the technique does not limit the deposition thickness. This is due to the fact that the technique requires two phases: deposition of a highly porous layer of silica and sintering of the layer in order to obtain a compact silica layer.
The sintering process has been thoroughly studied for the fiber fabrication. In the case of planar waveguides, the process is necessarily different since there is the presence of a substrate, and therefore large temperature changes during the sintering process generate constraints within the layer, which must be reduced at maximum. Different groups have realized such waveguides on a silicon substrate. The disadvantage of silicon substrates is that the silicon index of refraction is particularly high, and part of the light propagating in the silica core is evanescently captured by the silicon substrate and absorbed there. At low power, this results in losses. At high power, it might also result in substrate heating and finally device failure.
It is therefore more convenient to have a whole glass or crystal technology. This has been realized in some cases by bonding planar wafers together (D. P Shepherd et al, High power planar dielectric waveguide lasers, Journal of Physics D, 2001, vol. 34, page 2420). However this technique does not use microelectronics techniques and therefore does not lead to cost reduction. Moreover, it is more limited in the design options.